The general goal of this grant is to understand the cellular and molecular mechanisms involved thymocyte-thymic stromal interactions that regulate thymic stromal and thymocyte cytokine production, proliferation and maturation. Work in this grant over the past four years has led to the discovery of: 1) a novel molecule that mediates thymocyte-thymic stromal interactions (ALCAM), and 2) evidence of a role for the early thymocyte marker CD7 in T cell activation. The CD6 ligand, Activated Leukocyte Cell Adhesion Molecule (ALCAM), is a 100kd Ig superfamily molecule expressed on human TE cells and thymic fibroblasts that mediates thymic epithelial- thymocyte binding. Analysis of the ALCAM cDNA sequence demonstrated that ALCAM is the human homologue of the BEN/DM-GRASP chicken protein that is involved in growth, adhesion, and directed migration of motor neurons. In our studies of CD7, we have demonstrated human and murine CD7 promoter homology with murine Thy-1, prepared the first CD7 anti-mouse mabs, constructed CD7 transgenic mice, constructed CD7 homologous recombinant (gene knockout) mice, and demonstrated association of the p85 subunit of PI-3 kinase with the cytoplasmic domain of CD7 after ligation of the CD7 extracellular domain in Jurkat T cells. The ligand for ALCAM, CD6, and the CD7 molecule are both important signalling molecules of T cells that are expressed on CD4-CD8-CD3- T cell precursors in thymus. Thus, this grant will focus on the study that the roles of two Ig superfamily domain members, the CD6 ligand (ALCAM) on TE cells and thymic fibroblasts, and CD7 on T cells, play in human T cell development and function.